Method and apparatus for measuring in-place soil density and moisture content

ABSTRACT

A method and apparatus for measuring in-place soil density and moisture content. A cylindrical cell is disclosed which may be used to measure the density and the dielectric constant of a soil sample placed within the cylindrical cell. Also disclosed is a multiple rod probe which is designed to contact spikes driven into the ground to measure the in-place dielectric constant of soil. The multiple rod probe includes adjustable studs which ensure complete contact with the spikes. Both measurements are performed using time domain reflectometry. The present invention develops equations for determining the density of the soil in-place from the measured dielectric constant of the soil in-place and the measured density and dielectric constant of the soil in the cylindrical cell.

This invention was made with Government support from the IndianaDepartment of Transportation/Federal Highway Administration. TheGovernment has certain rights in the invention.

This application is a divisional of application Ser. No. 08/705,606,filed Aug. 30, 1996 now U.S. Pat. No. 5,801,537 which claims the benefitof U.S. provisional application Ser. No. 60/003,021, filed Aug. 30,1995.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to scientific measurementmethods and apparatus and, more particularly, to a method and apparatusfor measuring in-place soil density and moisture content.

BACKGROUND OF THE INVENTION

For the past 20 years, time domain reflectometry has been used tomeasure the volumetric moisture content of soils (volume of moisture perunit volume of soil), mostly in the field of soil science. As shown inFIG. 1, these measurements involved the insertion of a probe 10comprising a central rod 12 and two or more peripheral rods 14 into thesoil 16 to be measured. The peripheral rods 14 (which are preferablythree in number) are spaced equidistant from the central rod 12 andequidistant from each other. A coaxial transmission line 18 is thencoupled to the structure with the center conductor of the coaxial cable18 coupled to the center rod 12 and the exterior shield (outerconductor) of the coaxial cable 18 coupled to each of the peripheralrods 14. In this way, the peripheral rods 14 simulate the effects of acontinuous outer coaxial shield in the soil 16, without the requirementof attempting to drive a cylindrical probe into the soil 16. Time domainreflectometry analysis equipment 20 is then coupled to the coaxial cable18, and the reflections of high frequency electrical signals from thesoil 16 are measured using the analysis equipment 20. These reflectionswill change in predictable ways depending upon the dielectric constantof the soil 16, which has been found to be strongly correlated with thevolumetric moisture content of the soil 16. Therefore, time domainreflectometry has been established as a viable tool for measuringvolumetric moisture content of a soil.

The prior art probes such as those illustrated in FIG. 1 are intendedfor permanent installation at a measurement location with periodicmeasurements being made through the probe 10 over a period of time.Physically, the prior art probes 10 are not rugged enough to withstandrepeated insertion into and extraction from hard soils. The prior artprobes 10 are not suitable as portable probes to be used for rapidinsertion and removal following one-time soil measurement at a varietyof locations within a soil field to be measured. There is therefore aneed for a probe design which is rugged enough to withstand repeatedinsertions and extractions from dense soil, thereby facilitating thetaking of one-time measurements at multiple locations. The presentinvention is directed toward meeting this need.

Although time domain reflectometry techniques are useful in measuringvolumetric moisture content of soils, they cannot be presently used tomeasure gravimetric moisture content of soils (weight of moisture perunit weight of soil solids). Many applications in geotechnicalengineering require a knowledge of the gravimetric moisture content ofsoil. In order to convert from the volumetric moisture content measuredby time domain reflectometry to the gravimetric moisture content, it isnecessary to know the density of the soil. There are several prior artmethods for measuring in-place density in moisture content of soils,such as the sand-cone method, the rubber balloon method and the drivetube method. These methods are difficult and time consuming and areusually accompanied by the oven drying method of measuring moisturecontent in order to measure in-place dry density and moisture content ofthe soil. The oven drying method of measuring moisture content requiresa significant waiting time before measurement results are available.Another method, the nuclear method of measuring in-place soil moisturecontent and density requires extensive calibration. Moreover, thenuclear method is potentially hazardous because it utilizes radioactivematerials. There is therefore a need for a technique to measure in-placegravimetric moisture content and density quickly, precisely, andpreferably in a non-destructive manner. The present invention isdirected toward meeting this need.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for measuringin-place soil density and moisture content. A cylindrical cell isdisclosed which may be used to measure the density and the dielectricconstant of a soil sample placed within the cylindrical cell. Alsodisclosed is a multiple rod probe which is designed to contact spikesdriven into the ground to measure the in-place dielectric constant ofsoil. The multiple rod probe includes adjustable studs which ensurecomplete contact with the spikes. Both measurements are performed usingtime domain reflectometry. The present invention develops equations fordetermining the density of the soil in-place from the measureddielectric constant of the soil in-place and the measured density anddielectric constant of the soil in the cylindrical cell.

In one form of the invention an apparatus for measuring a moisturecontent of a soil sample is disclosed, comprising a container having aclosed first end, an open second end and a substantially cylindricalconductive sidewall defining an interior volume adapted to receive thesoil sample; a template having a central opening therethrough that iscoaxial with a longitudinal axis of the sidewall, the template adaptedto be removably mounted to the open second end; a hand penetrometerhaving an elongated rod sized to be inserted through the templatecentral opening and into the soil sample such that a hollow shaft iscreated along the longitudinal axis; and a cap adapted to be removablymounted to the open second end after the hollow shaft has been createdand after the template has been removed from the open second end, thecap comprising: a conductive head adapted to contact the sidewall; aconductive central rod; and a first annular non-conductive spacercoupling the conductive head to the conductive central rod; wherein thecentral rod substantially fills the hollow shaft when the cap is mountedto the open second end; wherein the assembled container, soil sample andcap form a coaxial transmission line wherein the soil sample serves as adielectric.

In another form of the invention a method of preparing a soil sample formeasurement of a moisture content of the soil sample is disclosed,comprising the steps of: (a) providing a container having a closed firstend, an open second end and a substantially cylindrical conductivesidewall defining an interior volume adapted to receive the soil sample;(b) removably mounting a template to the open second end, wherein thetemplate has a central opening therethrough that is coaxial with alongitudinal axis of the sidewall; (c) inserting an elongated rodthrough the template central opening and into the soil sample such thata hollow shaft is created along the longitudinal axis; (d) removing theelongated rod; (e) removing the template; (f) providing a cap,comprising: a conductive head adapted to contact the sidewall; aconductive central rod; and an annular non-conductive spacer couplingthe conductive head to the conductive central rod; and (g) mounting thecap to the open second end such that the central rod substantially fillsthe hollow shaft, wherein the assembled container, soil sample and capform a coaxial transmission line wherein the soil sample serves as adielectric.

In another form of the invention an apparatus for measuring moisturecontent of an in-place soil sample is disclosed, comprising a templatehaving a central hole therethrough and a plurality of peripheral holestherethrough, the plurality of peripheral holes being substantiallyequidistant from the central hole; a plurality of spikes adapted to bedriven through the central and peripheral holes of the template and intothe soil sample; and a probe head, comprising: an annular conductivebody; a plurality of conductive peripheral studs mounted to a bottomsurface of the body; a conductive central stud; and an annularnon-conductive insert coupling the body to the central stud; wherein afirst spacing between the central stud and the peripheral studs issubstantially the same as a second spacing between the central hole andthe peripheral holes, such that each stud is aligned with a respectivespike when the probe head is placed over the spikes after they have beendriven into the soil sample.

In another form of the invention an apparatus for measuring moisturecontent of an in-place soil sample is disclosed, comprising an annularconductive body; a plurality of conductive peripheral studs adjustablymounted to a bottom surface of the body, such that an extension of eachof the peripheral studs away from the body may be adjusted; an annularnon-conductive insert coupled to the body; a conductive central studadjustably mounted to the insert such that an extension of the centralstud away from the insert may be adjusted.

In another form of the invention a method of installing an apparatus formeasuring a dielectric constant of an in-place soil sample is disclosed,comprising the steps of: (a) providing a template having a central holetherethrough and a plurality of peripheral holes therethrough, theplurality of peripheral holes being substantially equidistant from thecentral hole; (b) laying the template on a surface of the in-place soilsample; (c) driving a plurality of conductive spikes into the soilthrough the central hole and each of the peripheral holes of thetemplate; (d) removing the template; (e) providing a probe head,comprising: an annular conductive body; a plurality of conductiveperipheral studs mounted to a bottom surface of the body; a conductivecentral stud; and an annular non-conductive insert coupling the body tothe central stud; wherein a first spacing between the central stud andthe peripheral studs is substantially the same as a second spacingbetween the central hole and the peripheral holes, such that each studis aligned with a respective spike when the probe head is placed overthe spikes after they have been driven into the soil sample; (f) placingthe probe head onto the conductive spikes, such that each stud isaligned with a respective spike.

In another form of the invention a method of installing an apparatus formeasuring a dielectric constant of an in-place soil sample is disclosed,comprising the steps of: (a) providing a template having a central holetherethrough and a plurality of peripheral holes therethrough, theplurality of peripheral holes being substantially equidistant from thecentral hole; (b) laying the template on a surface of the in-place soilsample; (c) driving a plurality of conductive spikes into the soilthrough the central hole and each of the peripheral holes of thetemplate; and (d) removing the template.

In another form of the invention a method of installing an apparatus formeasuring a dielectric constant of an in-place soil sample is disclosed,comprising the steps of: (a) providing a probe head, comprising: anannular conductive body; a plurality of conductive peripheral studsmounted to a bottom surface of the body; a conductive central stud; andan annular non-conductive insert coupling the body to the central stud;wherein the central stud and the peripheral studs are arranged in afirst pattern; (b) driving a plurality of conductive spikes into thesoil, wherein the spikes are arranged in the first pattern; and (c)placing the probe head onto the conductive spikes, such that each studis aligned with a respective spike.

In another form of the invention a template for guiding an installationof spikes into an in-place soil sample in a predetermined pattern isdisclosed, comprising a plurality of template sections which fittogether to form the template, wherein a plurality of holes are formedthrough the template, each hole straddling an intersection between twoor more template sections; at least one hinge coupling the plurality oftemplate sections to one another; and a releasable coupling joining twoof the plurality of template sections to one another; wherein theassembled template forms the holes into the predetermined pattern,thereby allowing the spikes to be driven into the soil through theholes; and wherein the releasable coupling may be released after thespikes have been driven into the soil, thereby allowing each templatesection to be swung away from the spikes by rotating each templatesection about its attached hinge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a prior art probe for measuring thedielectric constant of an in-place soil sample.

FIGS. 2A-B illustrate various components of a cylindrical cell of thepresent invention.

FIG. 3 is a side elevational view of a first embodiment multiple rodprobe of the present invention, illustrated in use with a template andspikes of the present invention.

FIG. 4 is a cross-sectional view of the first embodiment multiple rodprobe of FIG. 3.

FIG. 5 is a bottom plan view of the first embodiment multiple rod probeof FIG. 3.

FIG. 6 is a first embodiment hinged template of the present invention.

FIG. 7 is a side elevational view of a releasable coupling of a firstembodiment hinged template of FIG. 6.

FIG. 8 is a top plan view of a conductive ring of the present inventionfor allowing the multiple rod probe of the present invention to be usedwith the cylindrical cell of the present invention.

FIG. 9 is a side elevational view of the conductive ring of FIG. 8.

FIG. 10 is a graph of gravimetric moisture content as calculated by Eq.6 and by a prior art oven dry method.

FIG. 11 is a graph of gravimetric moisture content as calculated by Eq.7 and by a prior art oven dry method.

FIG. 12 is a graph of gravimetric moisture content as calculated by Eq.18 and by a prior art oven dry method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The time domain reflectometry (TDR) technique measures the velocity ofan electromagnetic wave traveling through a transmission line (TL). Thisvelocity (v) is related to the dielectric constant of the insulatingmedium between the conductors of the transmission line as given by##EQU1## where c is the velocity of light in a vacuum and K is thedielectric constant of the medium.

A TDR probe 10 for measuring soil moisture content is actually atransmission line whose dielectric medium is soil when this probe isdriven into soil 16. This probe 10 is connected with the TDR instrument20 via a coaxial cable 18. The TDR device 20 sends a step pulse down thecable 18. When this signal reaches the beginning of the probe 10, aportion of the signal is reflected back to the TDR device 20. When therest of the signal reaches the end of the probe 10, another reflectionof the signal occurs. These two reflections cause two discontinuties inthe resulting signal displayed on the TDR device 20 screen. The timedifference between these two discontinuties is the time (t) required bythe signal to travel twice the length (L) of the probe in soil. So thewave propagation velocity in soil is ##EQU2## and the dielectricconstant of soil is (using Eq. 1)

    K=(ct/2L).sup.2                                            (3)

In commercial TDR instruments, the term ct/2 is reduced to an apparentlength (l) resulting in

    K=l.sup.2 /L.sup.2                                         (4)

Research over the last 20 years, mostly in soil science, has shown thatthe dielectric constant of soil is directly related to the volumetricmoisture content of soil. Volumetric moisture content (volume ofmoisture per unit volume of soil) is related with gravimetric moisturecontent (weight of moisture per unit weight of soil solids) as ##EQU3##where: w is the gravimetric moisture content of the soil, θ is thevolumetric moisture content of soil, ρ_(d) is the dry density of soiland ρ_(w) is the density of water. In geotechnical engineering, moisturecontent is usually measured in terms of gravimetric moisture content.

An empirical relationship between the dielectric constant and thevolumetric moisture content of soil is

    θ=-0.053+2.92×10.sup.-2 K-5.5×10.sup.-4 K.sup.2 +4.3×10.sup.-6 K.sup.3                              (6)

where θ is the volumetric moisture content of soil. Equation 6 is notappropriate for organic soils or heavy clays. It has been recommended touse a soil specific calibration equation where Eq. 6 does not providesufficiently accurate results. Recent studies have developed betterrelationships to correlate K-θ. It has been observed that there exists alinear relationship between θ and K⁰.5 (if the density of soil does notvary much) of the form

    K.sup.0.5 =c.sub.1 +c.sub.2 θ                        (7)

where c₁ and c₂ are constants for specific soil type. It has beenreported that c₁ and c₂ also depend on density of soil. Depending onsoil type and density, c₁ may vary between approximately 1.2 to 1.6 andc₂ may vary between approximately 7.8 to 9.7. The present invention usesEqs. 6 and 7 for measuring moisture content and discloses yet anotherequation for measuring moisture content hereinbelow.

The existence of a direct correlation between K⁰.5 and θ as expressed inEq. 7 is the theoretical basis for measuring in-place density andmoisture content. Using Eq. 5, gravimetric moisture content can becalculated from θ if the density of the soil is known, and density canbe calculated from θ if the gravimetric moisture content is known.

The in-place density of the soil, however, cannot be directly measured.The present invention solves this problem by correlating measurementsmade on a sample of the soil placed in the cylindrical cell (CC) 30 ofFIG. 2A with measurements made in-place with the multiple rod probe(MRP) 70 of FIG. 3.

The cylindrical cell 30 of FIG. 2A forms a coaxial transmission line formeasuring the moisture content of a sample of soil removed from theground. The cylindrical cell 30 includes a cylindrical tube 32 whichholds the soil sample and acts as the outer conductor of the coaxialtransmission line. The tube 32 is preferably formed from a thinconductive metal, such as stainless steel, and preferably includes alongitudinal cut which creates a small gap running the length of thetube 32. Before compacting soil in the tube 32, hose clamps or the like(not shown) are used to close the gap around the soil sample. The gapaids in removal of the soil from the tube 32 after testing is complete.Stability is provided during testing by a non-conductive base 34 havingtwo or more bolts 36 mounted thereto. The tube 32 is coupled to thebolts 36 by means of flanges 38 welded (or otherwise coupled) to thetube 32. Nuts (not shown) may be threaded onto the bolts 36 to bear downupon the flanges 38.

The cylindrical cell (CC) 30 further comprises a coaxial interface cap40 which also mounts onto the bolts 36 by means of flanges 42 welded (orotherwise) coupled to the interface cap 40. Nuts (not shown) may bethreaded onto the bolts 36 to bear down upon the flanges 42. Theinterface cap 40 is made of a conductive material, such as stainlesssteel. The lower central portion of the interface cap 40 contains anon-conductive insert 44, which is preferably formed of plastic. Insert44 further contains a second threaded metal insert 46 therein. A centerrod 48 screws into the threaded metal insert 46 and forms the centerconductor of the coaxial transmission line of the cylindrical cell (CC)30. The rod 48 could alternatively be threaded directly into the plasticinsert 44, but this reduces the durability of the cap 40 for repeateduse. The coaxial cable 18 of a TDR instrument 20 may be coupled to thetube 32 and center rod 48 by means of the connector 50 mounted to theinterface cap 40. The interface cap 40/rod 48 assembly is hereinafterreferred to as the coaxial assembly (CA) 52.

Insertion of the coaxial assembly (CA) 52 into hard soil samples isfacilitated by the hand penetrometer 54 and template 56 of FIG. 2B.After the soil sample has been compacted into the tube 32, the template56 is attached to the top of the tube 32 by means of the flanges 58 andthe bolts 36. Once mounted, the central hole 60 of the template 56 isdirectly over the longitudinal axis of the tube 32. The rod 62 of thehand penetrometer 54 is then inserted into the soil through the hole 60.Downward pressure on the rod 62 may be exerted by pressing upon thehandles 64. Once the rod 62 has been fully inserted through the hole 60,it is withdrawn completely. This leaves a central shaft through the soilsample in the tube 32 which is perfectly aligned to receive the centralrod 48 of the coaxial apparatus (CA) 52. The template 56 is then removedfrom the tube 32 and the coaxial apparatus (CA) 52 is mounted to thetube 32, completing the cylindrical cell (CC) 30. Use of the handpenetrometer 54 reduces the stress applied to the CA 52 with repeateduse. Use of the hand penetrometer 54 with the template 56 furtherassures that the central rod 48 of the CA 52 is inserted directly alongthe longitudinal axis of the CC 30, which is very important for accuratemeasurements.

Step-by-step procedures for performing a test with the CC 30 are asfollows:

1. The tube 32 is placed on the base plate 34 and fastened to it.

2. A representative quantity of soil sufficient to fill the tube 32 istaken from the in-place measurement site. For measuring in-fieldmoisture content, the soil is dug out from the ground where measurementis required. Then the soil is compacted in layers using a tamping rod.

3. The weight of the tube 32 with soil in it is measured using anelectronic balance to measure the wet density of the compacted soil inthe tube 32.

4. The template 56 is placed on top of the tube 32 and the handpenetrometer 54 is used to make a hole along the center line(longitudinal axis) of the soil specimen.

5. The template 56 is removed and the rod 48 of the CA 52 is pushedthrough the hole in such a way that it does not move laterally whilepushing. Fingers of one hand are placed on the surface around the rod 48to act as a guide for the rod 48 to resist any lateral movement of itwhile the other hand is used to push the CA 52 in the hole. Any air gapthat appears around the rod 48 near the top surface is closed bypressing the soil against the rod 48.

6. When the cap 40 of the CA 52 sits on top of the tube 32, connectionis made with the TDR equipment 20 and a reading is taken which gives thedielectric constant of the soil.

7. The rod 48 is removed from the tube 32 by pulling the cap 40 out ofthe soil sample.

This device and the above procedures provide the wet density of the soilin the tube 32 and its dielectric constant.

The multiple rod probe (MRP) 70 of the present invention is illustratedin FIGS. 3-5. The multiple rod probe (MRP) 70 is used to measure thedielectric constant (and hence the volumetric moisture content) of anin-place soil sample. The design of the multiple rod probe (MRP) 70 issuch that it separates the conducting rods that are driven into the soilfrom the interface cap which is coupled to the TDR equipment 20. Inconventional probes, such as the probe 10 of FIG. 1, the conducting rods12, 14 are permanently connected to the interface cap. As illustrated inFIG. 3, the connection between the rods and the interface cap is notpermanent with the multiple rod probe (MRP) 70 of the present invention.The conducting rods 72 of the MRP 70 are driven into the soil 74 using atemplate 76 placed upon the surface of the soil 74 as a guide. It isextremely important that the rods 72 be driven into the soil 74 in sucha way that they fit the soil 74 tightly. Any gap around the rods 72 willgive erroneous results. The template 76 is preferably formed from woodor steel and includes guide holes through which the rods 72 are driveninto the soil 74 in a predetermined pattern. This pattern includes acentrally located rod and two or more peripherally located rods, allbeing equidistant from the central rod. The rods 72 are preferablycommon metal spikes, and extend into the soil 74 to a depth ofapproximately 9 inches. The template 76 is removed after the rods 72have been driven into the soil 74.

The multiple rod probe (MRP) 70 further includes an interface cap 78which is formed from a conductive material, such as stainless steel. Asshown in the cross-sectional view of FIG. 4 and the bottom view of FIG.5, the cap 78 includes a central annular non-conductive insert 80, whichis preferably formed of plastic. The cap 78 has a plurality of threadedstuds 82 threadingly engaged therewith. The centrally located stud 82 isthreaded into the non-conductive insert 80, while the peripheral studsare threaded directly into the conductive portion of the cap 78. Acoaxial connector 84 is mounted to the cap 78 in such a way that theouter conductor of the coaxial connector 84 contacts the conductiveportion of the cap 78 (and therefore the peripherally located studs 82),while the center conductor of the connector 84 contacts the centrallylocated stud 82 but is insulated from the conductive portion of the cap78. The connector 84 is coupled to a TDR instrument 20 by means of acoaxial cable 18.

In order to use the multiple line probe (MRP) 70, the template 76 isplaced on the surface of the ground at a location where it is desired tomeasure the dielectric content of the soil 74. The spikes 72 are thendriven into the soil 74, using the template 76 as a guide. Once thespikes 72 have been driven into the soil 74, the template 76 is removedand the cap 78 is positioned on top of the spikes 72 such that the studs82 make contact with each of the spikes 72. If any of the studs 82 donot touch the corresponding head of the spikes 72, the length of thestud 82 may be adjusted by turning the stud 82 until proper connectionis established. Thus configured, the MRP 70 is, in fact, a combinationof two transmission lines. The first segment has air as the dielectricmedium, and the second segment has soil as the dielectric medium. As theair has a very low dielectric constant compared to moist soil, adistinctive discontinuity in the reflected signal will appear at theair-soil interface. This will facilitate proper measurement of thedielectric constant of the soil 74.

The present invention achieves an advantage over the prior art probe 10of FIG. 1 by completely separating the in-ground spikes 72 from themeasurement cap 78. The robust spike 72 may be repeatedly driven intohard soil without creating any wear and tear upon the cap 78. In fact,the spike 72 may be driven into soils (using a hammer) into which theprior art probe 10 could not be inserted. The MRP 70 thereforefacilitates making many measurements of soil at different locations,whereas the prior art probe 10 of FIG. 1 is normally permanently mountedat a location and repeated measurements are made at the same location.

A preferred embodiment of the template 76 for use with the MRP 70 isillustrated in plan view in FIG. 6. The template 76 of FIG. 6 isintended for use with an MRP having one central probe and threeperipheral probes. Those skilled in the art will recognize that thedesign of the template 76 of FIG. 6 may be readily altered toaccommodate an MRP 70 having a fewer or greater number of peripheralprobes. The template 76 is formed from three identical pieces ofstainless steel 86, 88 and 90. The pieces 86-90 fit together in themanner shown, and define a central hole 92 and three peripheral holes 94therebetween. The piece 86 is coupled to the piece 88 by means of ahinge 96, and the piece 88 is coupled to the piece 90 by means of asecond hinge 96. The hinges 96 allow the pieces 86-90 to be unfoldedaway from the spikes 72 after they have been driven into the soil 74.Such removal of the template 76 is necessary when the heads of thespikes 72 have a greater diameter than the holes 92 and 94. The template76 is held in its closed position by means of a block 98, a screw 100and a screw 102. As shown in the end view of FIG. 7, the block 98 isrotatably mounted to the piece 90 by means of the screw 100. The screw102 is engaged by the block 98 by means of a slot 104. The block 98 maybe disengaged from the screw 102 by rotating the block 98counterclockwise about the screw 100. The slot 104 allows passage of theblock 98 around the screw 102. Once the block 98 has been disengagedfrom the screw 102, the pieces 86-90 may be folded away from each otherin order to release the template 76 from the spikes 72.

In an alternative embodiment of the present invention, the cap 78 of theMRP 70 may be used to replace the CA 52 of the cylindrical cell (CC) 30.In order to do this, the hand penetrometer 54 and the template 56 areused to create a central bore through the soil sample, as with the firstembodiment. A spike 72 is then inserted into this central bore. Thespike 72 will make contact with the central stud 82 of the cap 78. Inorder to provide contact between the peripheral studs 82 of the cap 78and the outer surface of the tube 32, the ring 106 is placed over thetop of the tube 32. The ring 106 is made from a conductive material,such as stainless steel. The ring 106 contains a central opening 108which allows the central stud 82 of the cap 78 to contact the spike 72.However, the ring 106 further includes a flat annular section 110 whichis wide enough to make contact with the peripheral studs 82 of the cap78. The ring 106 therefore provides conductive contact between theperipheral studs 82 of the cap 78 and the shell of the tube 32. By usingthe cap 78 as the measurement device for the cylindrical cell (CC) 30,instead of the purpose-built coaxial assembly (CA) 52, only a singlemeasurement device is required for both the cylindrical cell (CC) 30 andthe multiple rod probe (MRP) 70. Furthermore, use of the cap 78 withboth devices facilitates uniformity among measurements made with bothdevices.

Therefore, use of the multiple rod probe (MRP) 70 allows measurement ofin-place soil dielectric constant (which yields θ of the soil in-place).Use of the cylindrical cell (CC) 30 measures θ of the soil sample withinthe cylindrical cell as well as the wet density of the soil within theCC 30. The gravimetric moisture content w of the soil in the CC 30 maythen be computed. As discussed hereinbelow, this gravimetric moisturecontent of the soil within the CC 30 may be used to calculate thein-place soil density from the measured in-place volumetric moisturecontent, θ.

The coaxial apparatus CA 52 measures θ in the cylindrical cell (CC) andthe multiple rod probe (MRP) 70 measures θ of soil in-place. Thegravimetric moisture content of soil in the CC 30 is measured using themeasured density of soil in the CC 30. This gravimetric moisture contentcan be used to calculate the in-place density from the measured in-placevolumetric moisture content, θ. From the Eq. 5, moisture content, w, canbe expressed in terms of total density, ρ_(t), and ##EQU4## So, ##EQU5##where: w_(cc), θ_(cc), and (ρ_(t))_(cc) are the gravimetric moisturecontent, volumetric moisture content and total density of the soil inthe CC 30, respectively. Also ##EQU6## where: w_(insitu), θ_(insitu),and (ρ_(t))_(insitu) are the gravimetric moisture content, volumetricmoisture content and total density of the soil in-place, respectively.Since the soil in the CC 30 is quickly taken from in-place, it can beassumed that

    w.sub.insitu =w.sub.cc =w                                  (11)

Therefore, ##EQU7## Equating right hand sides of Eqs. 9 & 10, we get##EQU8## Equations 12 and 13 give in-place moisture content and totaldensity of soil. In-place dry density can be calculated from totaldensity using the measured moisture content as ##EQU9## where(ρd)_(insitu) is the dry density of in-place soil. The unit weight ofsoil can be calculated from the density as

    (Υ.sub.t).sub.insitu =(ρ.sub.t).sub.insitu g   (15)

and

    (Υ.sub.d).sub.insitu =(ρ.sub.d).sub.insitu g   (16)

where (Υ_(t))_(insitu) and (Υ_(d))_(insitu) are the total unit weightand the dry unit weight of in-place soil respectively, and g is theacceleration due to gravity. The accuracy of measuring total densitywould be better than that of dry density as evident from Eq. 14. Anyerror in measuring gravimetric moisture content, w, using the CC 30contributes for the higher error in measuring dry density compared tothe error involved in measuring total density.

To assess the performance of Eq. 13 in measuring in-place density, it isnecessary to compare it with conventional measured density. Also it isnecessary to test the performance of Eq. 13 for many possibleparameters. Laboratory experiments using cylindrical cells (CC) 30 canbe conducted to achieve these goals. If two soil specimens are preparedat the same moisture content, w, in two cylindrical cells (CC) 30, thenthe density of one of the specimens can be measured using the density ofthe other specimen using ##EQU10## The density thus measured can becompared with the conventionally measured density of the soil specimen.

An experimental program was undertaken to assess the performance of theCC 30 in measuring moisture content, and to develop the procedure formeasuring in-place density of soil. The development of the procedure fordetermining in-place density required determination of dielectricconstants of soil specimens having the same moisture contents but havingdifferent dry densities. Using the CC 30 was most easy and ideal forthat purpose. Experiments with the CC 30 to develop K⁰.5 -ρ_(d)relationships provided the opportunity to assess the performance of theCC 30 in measuring in-place moisture content and to develop theprocedure for measuring density. A wide range of parameters, i.e. soiltype, moisture content, density, etc. were covered in the experimentsusing the CC 30. To assess the performance of the developed procedurefor measuring density, field experiments were conducted using the MRP 70and the CC 30. The prior art sand-cone method was used to measurein-place density to compare with the in-place density measured by thepresent technique. As there is no exact prior art method of measuringin-place density to correctly assess the performance of the method ofthe present invention, it was necessary to conduct experiments in thelaboratory under simulated field conditions to correctly assess theperformance of measuring in-place density.

The CC 30 shown in FIG. 2 was used for laboratory experiments. Importantdimensions of the device were: effective length of probe, L=28.6 mm;diameter of probe rods, d=4.76 mm; inside diameter of CC, D_(i) =72.6mm; outer diameter of CC, D_(o) =76.2 mm.

The main steps for the laboratory experiment were:

1. Prepare the soil at a specified moisture content.

2. Compact the soil in two coaxial cells (CC's) 30 in a way that soil ineach cell has a different density. Specimens should be prepared in a waythat the moisture content of each specimen remains the same.

3. Measure the dielectric constants of the specimens using the CA 52.

4. Measure the wet-densities of specimens in each cell 30. Take a samplefrom each cell 30 for measuring moisture content by the prior art ovendrying method. Use oven drying moisture content to calculate drydensities.

5. Compute gravimetric moisture content of soil in each cell 30 usingthe measured dielectric constant and wet density. Compute dry density(with Eq. 14) using this gravimetric moisture content.

6. Use the dry density of one of the specimens measured in step (5) tomeasure the dry density of the other specimen using Eq. 17. Compare thisdensity with that calculated in step (4).

7. Compare the gravimetric moisture content computed in step (5) withthe oven drying moisture content.

8. Prepare additional pairs (at least four) of specimens with the soilprepared at the same moisture content used in step (1), but withdifferent compacting efforts. Densities of the specimens should coverthe whole range of normal densities (loosest to densest). Also thedifference in density between the specimens in each pair should coverall possible ranges. Repeat steps (1) to (6) for soil prepared at othermoisture contents to cover a wide range of moisture content.

                  TABLE 1    ______________________________________    SOIL CHARACTERISTICS                                   Range  Range           %         %      %      density                                          moisture    Soil Type           Sand      Silt   Clay   (Mg/m.sup.3)                                          content (%)    ______________________________________    Crosby till           5         15     80     1.05-1.55                                          12-27    Silt   0         100     0     1.2-1.6                                          4-24    Fine Sand           100        0      0      1.4-1.68                                          1.2-13.0    Kaolinite           0          0     100     1.0-1.45                                          6-28    Cherry L.           5         70     25      1.3-1.65                                          8-24    Soil    ______________________________________

For field experiments, the MRP 70 of FIG. 3 and the CC 30 of FIG. 2 wereused. The key dimensions of the MRP 70 were: effective length of probe,L=230 mm; diameters of probe rods, d=7.9 mm; spacings of the inner tothe outer rods, s=37 mm. The main steps for the field experiments were:

1. Use the MRP 70 to measure in-place dielectric constant of soil.

2. Dig out a sufficient quantity of soil from the place of in-placemeasurement and compact it in a number of CC's 30 at different densities(loosest to densest). Measure the dielectric constants of the soil ineach of these CC's 30. The soil to be used in each CC 30 should be takenuniformly over the depth of MRP 70 insertion.

3. Measure the wet density of the soil in the CC 30. Compute thevolumetric and the gravimetric moisture content of the soil in the CC 30from the dielectric constant and measured densities (using Eq. 6 and 9).Calculate the dry density of the soil with the calculated gravimetricmoisture using Eq. 14.

4. Relate the dry density of the soil in the CC 30 to the dry density ofthe soil in-place using the Eq. 13.

5. Measure the wet density of the soil at the place of in-placemeasurement by the prior art sand cone method.

6. Take a sample for measuring moisture content by the prior art ovendrying method. Compute the dry density from the wet density measured bythe prior art sand cone method. Compare this dry density with that foundin step (4).

7. Compare the moisture content measured by TDR method with thatmeasured by oven drying method.

The simulated field experiment was conducted in the laboratory using thesame devices as those that were used for the field experiments. The onlydifference was that the solid whose in-place density and moisturecontent was to be measured was the soil compacted in a large mold. TheMRP 70 was installed in the central area of the soil in the mold. Themain steps of the method were:

1. Prepare the soil at a desired moisture content and compact the soilin a big compaction mold (6 inch diameter, 9 inch height).

2. Use the MRP 70 to measure the in-place dielectric constant of thesoil. The length of the conductors 72 should be little more than 9 inch(about 10 inch).

3. Measure the wet density of the soil in the mold.

4. Take soil from the mold to measure the dielectric constant in the CC30. Measure the wet density of the soil in the CC 30 and computevolumetric moisture content (Eq. 6), gravimetric moisture content (Eq.9), and dry density (Eq. 14) of the soil. Compute the dry density of thesoil in the mold using this information (Eq. 17). Take a sample of soilfor measuring moisture content by the prior art oven drying method.

5. Repeat step (4) to perform at least 5 more tests. Each time compactthe soil in the CC 30 at a different density to cover a wide range ofdensities (loosest to densest).

6. Compute the actual dry density of the soil in the mold from themeasured wet density and the prior art oven dry moisture content.

7. Compare the measured dry density and the moisture content with theactual dry density and the prior art oven dry moisture content.

8. Repeat the whole procedure for soils prepared at other moisturecontents to cover a wide range of moisture content.

Eq. 6 and Eq. 8 were used to compute gravimetric moisture contents fromthe measured dielectric constants. A very good correlation (R² =0.96) isobtained between the actual and the measured moisture contents. Thestandard error of measurement is 0.013. The error involved in moisturecontent measurement using Eq. 6 is higher for clayey soil compared tocohesionless soil, as expected. These findings prove the validity of thedesign and procedures involved in measuring moisture content using thecylindrical cell (CC) 30.

To obtain a calibration equation of the form of Eq. 7, values of K⁰.5were plotted with actual volumetric moisture contents. The result ofregression analysis on this data is: K⁰.5 =1.5+8.4θ. To obtain soilspecific calibration equations, regression analyses were carried out foreach soil type measured in this study. Table 2 gives the coefficients ofregression analyses and values of R². The values of intercepts vary from1.14 to 1.58, values of slopes vary from 8.26 to 9.78, correlationcoefficients vary from 0.967 to 0.988. From these results it is clearthat a common calibration equation may not be applicable for all soils.Soil specific calibration equations are necessary for accurate results.Using these soil specific calibration equations, soil moisture contentswere back calculated. The standard error of measurement for this data is0.008, less than what was obtained using Eq. 6.

                  TABLE 2    ______________________________________    REGRESSION ANALYSIS: K.sup.0.5 = c.sub.1 + c.sub.2 θ    Soil Type             Intercept, c.sub.1                            Slope, c.sub.2                                     R.sup.2    ______________________________________    Crosby till             1.14           9.78     0.967    Silt     1.56           8.74     0.989    Fine Sand             1.59           8.263    0.987    Cherry   1.31           8.96     0.988    Lane    Kaolinite             1.59           8.30     0.989    ______________________________________

In order to measure density from the dielectric constant, it isnecessary to establish the relationship between K⁰.5 and density. FromEq. 7 it is seen that a linear relationship exists between K⁰.5 and θ.It is also likely that a linear relationship might exist between K⁰.5/ρ_(d) and w. Regression analysis for the K⁰.5 /ρ_(d) -w relationshipwas carried out for each soil type and the result is shown in Tab. 3.This table shows that an excellent linear relationship exists betweenK⁰.5 /ρ_(d) and w. Comparing Tab. 3 and Tab. 2, it is seen that a betterlinear relationship exists between K⁰.5 /ρ_(d) and w than the linearrelationship between K⁰.5 and θ for almost all soil types. From Tab. 3,it is seen that the values of intercept a are close to 1 for all soilsexcept kaolinite, for which is is 1.57. Similarly, the values of slope bare in the range of 8 to 9 except for kaolinite for which it is 6.79.From this data, it is obvious that a common calibration equation may notbe suitable for all soil types. There is excellent linear correlationbetween K⁰.5 /ρ_(d) and w for different types of soil. This observationshows that soil specimens having the same gravimetric moisture contenthave the same value for the ratio of K⁰.5 /ρ_(d). To see whether theration (K⁰.5 -c)/ρ_(d) has a better correlation with w than the K⁰.5/ρd-w correlation, they were plotted for different values of c rangingfrom 0 to 1. Table 4 shows the results of the regression analyses. It isseen that R² is not very sensitive to the values of c ranging from 0to 1. For simplicity, c=0 was chosen and K⁰.5 /ρd-w relationship wasused for measuring density. From these results it can be concluded that##EQU11## where, depending on soil type, a may vary from 0.95 to 1.6 andb may vary from 6.5 to 9.5.

                  TABLE 3    ______________________________________    REGRESSION ANALYSIS: K.sup.0.5 /ρ.sub.d = a + bw    Soil Type Intercept, a   Slope, b                                     R.sup.2    ______________________________________    Crosby till              0.978          9.17    0.947    Silt      1.102          8.178   0.993    Fine Sand 1.034          7.96    0.992    Cherry    0.993          8.65    0.995    Lane    Kaolinite 1.576          6.79    0.998    ______________________________________

                  TABLE 4    ______________________________________    REGRESSION ANALYSIS: (K.sup.0.5 -c)/ρ.sub.d = a + bw    c        a             b      R.sup.2    ______________________________________    0        1.007         8.081  0.985    0.3                           0.987    0.5      0.682         8.63   0.987    0.7                           0.986    1.0      0.356         8.447  0.982    ______________________________________

From Eq. 18 we get ##EQU12## where K₁, K₂ and ρ_(d1), ρ_(d2) are thedielectric constants and dry densities of two soil specimens having thesame moisture content w. Equation 19 can be written as

    R.sub.K =R.sub.d                                           (20)

where R_(K) and R_(d) are the ratio of refractive indices (K⁰.5 's) andratio of densities (dry or total), respectively. Equation 18 may be usedfor measuring gravimetric moisture content as ##EQU13## Equation 21 wasused with the appropriate values of a and b taken from Table 4 forcomputing w. Measured and actual moisture contents were compared. Thestandard error of measurement is 0.0075, which is less than thatobtained using Eq. 6 or Eq. 7. The data are given in Tab. 5.

                  TABLE 5    ______________________________________    REGRESSION ANALYSIS: w.sub.measured = a + bw.sub.actual    Eq. used    a     b          R.sup.2                                      S.E. (%)    ______________________________________    Eq. 6       0     0.97       0.96 1.21    Eq. 7       0     1.00       0.985                                      0.784    Eq. 16      0     0.999      0.985                                      0.750    ______________________________________

Dielectric constants of soil specimens having the same w were measuredin the laboratory using the CC 30. Eq. 19 was used to compute thedensity of one specimen using its dielectric constant and the densityand the dielectric constant of another specimen. Since dry density iscomputed from the measured total density and w, any error in measuring wwould affect accuracy of measuring density.

To correctly assess the performance of Eq. 19 in measuring density, itwould be better to compute total density and compare it with actualtotal density, thus isolating the effect of w on density measurement.Measured and actual total densities were plotted against one another andshowed reasonably good correlation between the measured and actualdensities; slope very close to 1, intercept very close to zero, and highcorrelation coefficient. The standard error of measurement is 0.0307Mg/m³. The gravimetric moisture contents were used to compute the drydensities and these were compared with the conventionally measured drydensity. The standard error is 0.032 Mg/m³, slightly higher than thatfor total density, because of the error involved in measurement ofgravimetric moisture content.

Plots of measured vs. actual total densities showed that there isscatter in the data. To increase the accuracy of the densitymeasurement, it is necessary to identify the factors that affect thedensity measurement accuracy. Probable factors that might affect theaccuracy of measurement are: R_(d), moisture content, density, etc.Percent error in density measurement was plotted with R_(d) and did notshow any remarkable trend of influence of R_(d) on percent error. Butfor some soil types, high R_(d) (R_(d) >1.1) may increase the errorinvolved in the density measurement. Percent error was plotted withmoisture content and, as expected, accuracy increases with increase inmoisture content.

The reason for higher error involved in measuring dry density from totaldensity is because of the error involved in measuring moisture content.The additional error in measuring dry density compared to that of totaldensity would be lower at higher moisture contents and when the errorinvolved in moisture content measurement is small.

The procedure developed was used to measure in-place density andmoisture content using the MRP 70 and the CC 30. Because it is difficultto conduct field experiments over widely varying moisture content anddensity conditions for a given soil, experiments were first conducted inthe laboratory under simulated field conditions. The soil used for thesimulated field experiment was the Cherry L. Soil. Densities weremeasured using Eq. 19. The measured density and the conventionallymeasured density were plotted. The measured and the actual densityfollow a 1:1 line. The standard error of measurement is only 0.020Mg/m³, much less than what was obtained from the laboratory experiments.Percent error in density measurement was plotted with R_(d) and w andthis data showed that the maximum error in density measurement is about3%. No noticeable trend of influence of R_(d) on measurement accuracy isobserved. But the error of measurement reduces as w increases. Themeasured gravimetric moisture contents using three different equationsare shown in FIGS. 10, 11 and 12. Eq. 6 gives the least accurate result(FIG. 10) compared to Eq. 7 plotted in FIG. 11 and to the new Eq. 18,which gives the best result (FIG. 12) with slope exactly equal to 1,intercept equal to 0 and R₂ of 0.995. Dry densities computed usingmeasured moisture contents using the new equation (as shown in FIG. 12)showed a standard error of measurement of dry density to be a very low0.021 Mg/m³. These results prove the validity of the procedure and Eq.18 developed for measuring density and moisture content.

Field experiments were conducted at a local field site know as CherryLane (the site soil was identified as Cherry L. Soil in Tab. 1). Densitywas compared with the density measured by the prior art sand-conemethod. The maximum variation of the measured density using the presentmethod is about 3%. Over a wide range of R_(d), the measurement accuracyis consistent irrespective of the value of R_(d). These results provethe validity of the procedure developed in the laboratory for measuringin-place density and moisture content.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

What is claimed is:
 1. An apparatus for measuring moisture content of anin-place soil sample, comprising:a template having a central holetherethrough and a plurality of peripheral holes therethrough, theplurality of peripheral holes being substantially equidistant from thecentral hole; a plurality of spikes adapted to be driven through thecentral and peripheral holes of the template and into the soil sample;and a probe head, comprising:an annular conductive body; a plurality ofconductive peripheral studs mounted to a bottom surface of the body; aconductive central stud; and an annular non-conductive insert couplingthe body to the central stud; wherein a first spacing between thecentral stud and the peripheral studs is substantially the same as asecond spacing between the central hole and the peripheral holes, suchthat each stud is aligned with a respective spike when the probe head isplaced over the spikes after they have been driven into the soil sample.2. The apparatus of claim 1, wherein the conductive central stud isthreadingly coupled to the insert, such that an extension of theconductive central stud away from the insert may be adjusted.
 3. Theapparatus of claim 1, wherein the peripheral studs are threadinglycoupled to the body, such that an extension of each of the peripheralstuds away from the body may be adjusted.
 4. The apparatus of claim 1,further comprising:a coaxial electrical connector having an outerconductor and an inner conductor separated by a non-conductive spacer,wherein the outer conductor is electrically coupled to the conductivebody and the inner conductor is electrically coupled to the conductivecentral stud.
 5. An apparatus for measuring moisture content of anin-place soil sample, comprising:an annular conductive body; a pluralityof conductive peripheral studs adjustably mounted to a bottom surface ofthe body, such that an extension of each of the peripheral studs awayfrom the body may be adjusted; an annular non-conductive insert coupledto the body; a conductive central stud adjustably mounted to the insertsuch that an extension of the central stud away from the insert may beadjusted.
 6. The apparatus of claim 5, wherein the central stud isthreadingly coupled to the insert.
 7. The apparatus of claim 5, whereinthe peripheral studs are threadingly coupled to the body.